The present invention relates to an application device for applying at least a solder resist onto both surfaces of a printed circuit board that are in particular provided with structurized printed conductors, the application device comprising at least two application bodies that can be assigned to the corresponding surface of the board and are rotatably supported.
In such an application device which is known in practice, an upper surface and a lower surface of the board are coated, for instance, by two printing rollers with solder resist applied to the outer surface thereof. The coating with the solder resist serves to protect all parts on the printed circuit board, except for those in which electric or electronic components are to be soldered.
The known application device has the disadvantage that the working speed is relatively small with such printing rollers and that the production of the printing rollers as well as the application of solder resist to an outer circumference of the rollers are relatively troublesome and difficult at the same time. Moreover, it is relatively difficult to control the thickness of the applied solder resist.
It is therefore the object of the present invention to improve an application device of the above-mentioned type in such a manner that said device has a simplified structure while allowing a more rapid and well-reproducible application, in particular, of a solder resist at both sides of a printed circuit board at the same time.
This object is achieved in that the application bodies are two screen printing drums arranged on both sides of an essentially horizontal transport path, wherein a transport means comprises conveyor belts along the transport path and the transport means further comprises pressing means adapted to be pressed onto a surface of the board for holding the printed circuit board on the conveyor belts.
Thanks to the arrangement of the application bodies at both sides, solder resist, in particular, is applied at the same time. Thanks to the horizontal transport path the solder resist will adhere to the surfaces of the board in an optimum manner, whereby the coating can simultaneously be applied to both surfaces of the printed circuit board in a uniform and constant manner due to the use of the screen printing drums. Moreover, a high speed can be achieved during application through the use of the screen printing drums, and the application device according to the invention can be constructed and used in a simplified manner at the same time and is thus less expensive than the prior-art application device comprising printing rollers.
Moreover, thanks to the simultaneous, uniform and constant application, in particular of solder resist, both surfaces of the printed circuit board can be dried at the same time and during the same period of time, thereby avoiding different degrees of sensitivity during subsequent exposure and corresponding development for the further structurization of the surfaces of the printed circuit board.
Moreover, thanks to the essentially horizontal transport path, known and already well-established devices can be used for the supply to and the discharge from the screen printing drums. The costs of the application device are thereby further reduced.
To transport and hold the printed circuit boards in an easy manner, the transport means comprises conveyor belts extending along the transport path as well as pressing means which can be pressed onto a surface of the board for holding the printed circuit board on the conveyor belts. Preferably, the pressing means press against an upper surface of the board opposite to the conveyor belts.
For the use of printed circuit boards having different dimensions and, in particular, different thicknesses, and for improved accessibility of the screen printing drums for repair, cleaning, or the like, at least one screen printing drum can be adjustably supported essentially in a direction perpendicular to the transport direction. In cases where only one screen printing drum is adjustably supported, the structure of the application device is further simplified.
The printed circuit boards are transported to the printing gap formed by the two screen printing drums, along their essentially horizontal transport path. To be able to print, if possible, on the whole upper and lower surface of the board, the horizontal transport path may be defined by the transport means which supports and/or holds at least edge portions or edges of the printed circuit board. On account of the small weight of the printed circuit board it is e.g. sufficient when a corresponding transport means presses from the outside onto the opposite edges of the printed circuit board that are positioned in parallel with the transport means, and when the transport means moves the board. The transport means can also act on the upper and lower surface of the board within a small edge portion of the printed circuit board as long as solder resist, for instance, need not be applied in said edge portion. Furthermore, transport means for edge portions and edge of the printed circuit board can be used in combination simultaneously or successively along the transport path.
Further possible configurations of the transport means and combinations thereof are obvious.
A transport means may e.g. be formed by transport wheels which press onto an outer edge of the printed circuit board. Moreover, the transport means may be provided along the transport path with conveyor belts which support the edge portions of the printed circuit board and/or which press against the outer edge of the printed circuit board.
Further possible configurations of the transport means and combinations thereof are obvious.
In the case of conveyor belts assigned to edge portions on the lower surface of the board, it may turn out to be an advantage when for a positionally accurate supply which is synchronized with the rotation of the screen printing drum, the transport means comprises pressing means adapted to be pressed onto the upper surface of the board for holding the printed circuit board on the conveyor belts.
Such pressing means may e.g. be pressing elements which can be assigned to an edge portion of the upper surface of the board. Moreover, the pressing means may comprise pressing rollers which can e.g. extend over the entire width of the printed circuit board in a direction perpendicular to the transportation direction. Such pressing means are arranged at least along the transport path that feeds the printed circuit boards to the screen printing drums. With a corresponding configuration or arrangement of the pressing means, the latter may also be arranged along the transport path which serves to transport the printed circuit boards away from the screen printing drums. It must here only be ensured that the pressing means do not remove the solder resist, which has e.g. been applied, or that said resist has not already dried before. To this end a corresponding drying means may e.g. be arranged directly along the transport path downstream of the screen printing drums.
The pressing rollers may preferably be supported in a pivotable manner while being adapted to be pressed onto the upper surface of the board in spring-biased fashion. The pressing rolls are preferably supported laterally from the transport path.
To apply solder resist to an increased number of printed circuit boards in the same or also a different manner, a plurality of transport paths for printed circuit boards may be arranged side by side for the parallel supply of printed circuit boards to the screen printing drums. The screen printing drums may here be configured in the area of the various transport paths in a similar manner, so that each of the printed circuit boards supplied in parallel fashion are provided with solder resist in a similar manner. The screen printing drums may also be configured along their rotational axis differently for each transport path, so that different patterns of solder resist can simultaneously be applied to printed circuit boards which are supplied in parallel fashion. Likewise, the printed circuit boards which are supplied in parallel fashion to the screen printing drums may e.g. be provided with the same coating on the upper surface of the board and with different coatings on the lower surface of the board, or vice versa. Moreover, it is possible to print each printed circuit board at one side or both sides repeatedly and in successive fashion with the help of a screen printing drum.
For a simple adjustment of the height of the at least one screen printing drum, a height adjusting means may be provided for automatically adjusting the height. It is thereby possible to lift or lower the adjustable screen printing drum relative to the other screen printing drum by the xe2x80x9cpush of a buttonxe2x80x9d.
In the simplest case the height adjusting means may comprise a height indicating means and a drive for lifting and lowering operations. The height indicating means may e.g. be a scale on which the respective height position of the adjustable screen printing drum can be read. The drive may e.g. comprise a gear which is driven by a motor and which rolls along a substantially vertically arranged toothed rack. Further drives can be implemented in an easy manner.
To further simplify the application device, a drive means may be assigned to both screen printing drums. Such a drive means may e.g. be a motor which is drivingly connected to a screen printing drum, and a drive belt arranged between the two screen printing drums for transmitting the rotational movement to the other screen printing drum. The drive means preferably comprises a motor and a speed controller for synchronizing the rotations of the screen printing drums. It is thereby ensured that the solder resist is applied along the printed circuit boards to the upper and lower surfaces thereof in a correct manner from the beginning to the end.
For instance, in order to easily apply different sizes of printed circuit boards and/or different patterns of solder resist to both sides of printed circuit boards, it is advantageous when the screen printing drums are supported in the application device in such a manner that they are exchangeable. As a result, it is e.g. possible to use screen printing drums having different diameters or different stencils.
To automate the operation of the application device and to simplify the same for a user, the application device may comprise an electronic control unit including at least one microprocessor and a storing means. With the help of the electronic control unit it is possible to request different application programs, and it is e.g. also possible to store differences in the stencils of different installable screen printing drums in the storing means. After a screen printing drum has been replaced, a corresponding application program can e.g. be requested automatically by means of the electronic control unit for different sizes of printed circuit boards or for a different stencil.
To support the screen printing drums in the application device in a simplified manner and to supply the material to be applied, in particular solder resist, in a simplified manner, each screen printing drum may be rotatably supported in a holding means which has adjustably supported thereon a dispenser and application means that can be assigned to an inner circumference of the screen printing drum. The dispenser and application means may contain a tank, or the like, for the material to be applied, with the material being dispensed in quantitatively metered amounts from said tank via a dispenser means and being applicable by means of the application device to an interior side of the stencil of the screen printing drums.